The present invention relates to measurement methods, and more particularly to apparatus for measuring the motion of a film in a film gate such as the projection gate of a motion picture projector, scanning gate in a film scanner, or recording gate in a film recorder.
During the projection of motion picture film the projected image on the screen can go in and out of focus resulting in momentarily blurred images reducing the picture quality on the projection screen causing discomfort to the viewer. There is a desire to maintain the sharpness of the projected image by keeping the film plane in focus and to eliminate the in and out of focus movement of the film which is objectionable to the viewer. It is known in the art that both thermal and mechanical induced motions will cause the film to go into and out of focus resulting in these undesirable effects.
During motion picture projection, successive film frames are transported intermittently. In a standard 24 frame per second 2 bladed shutter motion picture projection system, each film frame sees an approximately 40 ms cycle divided into four approximately 10 ms events. During the first event a new film frame is advanced and positioned into the film gate while the shutter is closed. During the second event the shutter opens rapidly and the stationary film frame is exposed to high intensity primary light source illumination. During the third event the shutter is closed rapidly and the film frame remains stationary. During the fourth event the shutter is again opened rapidly exposing the stationary film frame to high intensity primary light source illumination for a second time. After the fourth event the shutter is closed and the film is advanced to the next frame and the cycle is repeated.
The mechanical induced motion occurs during the transport event of the projection cycle. When the film is intermittently pulled through the gate, the film moves in and out of the plane defining the rest position. This in and out of the plane motion is called mechanical ringing and typically takes about 30 ms to settle. The thermal induced motion occurs during the second through fourth events. When the shutter is opened the film absorbs radiant energy from the high intensity primary light source and expands, which causes thermally induced motion. The film is heated during the first and second exposures (second and fourth events) and cools when the shutter is closed during the third event.
The current trend in the motion picture industry is to project onto larger screens, which require higher levels of illumination on the film in the projector gate. These higher levels of illumination amplify the undesirable thermal effects. These effects differ from one projector to the next, each projector having its own peculiar characteristics. This problem of thermal and/or mechanical deflection of film in a gate is also experienced in film scanners, film writers, photographic printers, and slide projectors.
Film scanners and film recorders are similar to film projectors in that they have film transport subsystems and high intensity illumination sources. Film projectors, scanners, and recorders require the film to be accurately positioned and include apparatus to maintain positional accuracy during operation. Radiant energy absorbed by the film from the illumination source causes the film to move or buckle. The film transport subsystems, at the component level, introduce mechanical vibrations into the film causing it to move or ring. The film motion characteristic profiles for thermal and mechanical induced motions are critical in order to maintain proper focus calibration with the rest of the optical system. The typical depth of focus requirements for precision film scanners, recorders, and projectors is less than four thousandths of an inch over the field of view. In order to properly design, assemble, align, calibrate, operate, and maintain precision film apparatus, real-time monitoring of the film dynamics is critical to meeting the performance criteria.
Film scanners transport film across a film gate. Typically, they illuminate from one side through the film onto a detector which is located on the other side of the film. The optical subsystem defines the planarity of the film at the film gate. Any motion, thermal and/or mechanical, which causes the film to displace beyond the depth of focus specification will result in a degraded, out of focus image. Real-time accurate film position data provides the opportunity to maintain in-spec performance and ensure optimal image scanning.
Film recorders typically transport film across a writing head in which a precision beam of light writes or exposes a latent image on the light sensitive film. The planarity of the film""s surface is required to be controlled within very tight tolerances in order to properly focus the beam on the surface. The resulting image quality is directly dependent on controlling film motion within the depth of focus of the optical system. Real-time accurate film position data provides the opportunity to maintain in-spec performance and ensure optimal image recording.
Thus, there is a need for real time instrumentation to characterize and enable understanding of the thermal and mechanical induced motions of the film as it is transported, illuminated and/or projected in the gate of a motion picture projector, film scanner or film recorder. The resulting knowledge will enable the design of improved motion picture projection systems, scanners and film recorders. For example, use of the instrumentation will enable better construction, alignment and calibration of motion picture projectors, film scanners and film recorders. In addition the same type of instrument can be utilized in the field for maintenance, repair and calibration of motion picture film projectors, film scanners and film recorder. The instrumentation can also be integrated into the motion picture film projector, film scanner or film recorder to provide active monitoring and real time feedback to servo control systems to maintain in-spec focus.
U.S. Pat. No. 3,471,225 issued Oct. 7, 1969 to Hutchison describes an automatic focusing motion picture projector which attempted to measure the buckle of the film at a frame adjacent to the projector film gate. The apparatus includes a light source arranged to project a beam of light at an obtuse angle onto the surface of the film and a split detector arranged to receive the reflected beam from the surface of the film. This arrangement does not actually measure the film deflection in the gate of the projector and does not provide a record of the deflected film as a function of time. The measurement looked for a desired amplitude and controlled a servo mechanism to keep the film at a desired distance. There was no means included to determine the magnitude of the film buckle.
U.S. Pat. No. 3,672,757 issued Jun. 27, 1972 to Szymber et al. discloses a slide projector means for maintaining focus in response to film deflection similar in principle to that of U.S. Pat. No. 3,471,225. U.S. Pat. No. 4,800,286 issued Jan. 24, 1989 to Brears discloses a measurement device for computing distance variations in a corrugated structure with a fiber optic lens probe and a detector sensing reflected light variations from peaks and valleys of the corrugated structure. U.S. Pat. No. 5,483,347 issued Jan. 9, 1996 to Hollmann discloses an apparatus for measuring distance wherein a light source from one fiber optic bundle is focused on a surface and reflected light therefrom is focused through another fiber optic bundle to a detector for calculation of the distance. In this apparatus the fiber optic probe is moved relative to the stationary object surface for calculating distance. U.S. Pat. No. 5,581,351 issued Dec. 3, 1996 to Marcus et al. discloses a method and apparatus for measuring thermal expansion of a rotating roller utilizing a pair of reflective fiber optic probes. U.S. Pat. No. 5,392,123 issued Feb. 21, 1995 to Marcus et al. describes an optical monitor for measuring a gap between a pair of rollers using a plurality of reflective pair fiber optic probes.
None of the above prior art will measure film deflection and dynamics in real time in the gate during the intense illumination from the primary light source present at the gate of a motion picture projector. Therefore, there is a need for an improved apparatus for measuring the out of plane motion of motion picture film traveling through the gate of a motion picture projector during illumination by high intensity optical sources. Furthermore, such an apparatus needs to be self-calibrating, robust, simple to install, be capable of making measurements in real time, and with high measurement reproducibility on the order of 1-micron measurement repeatability. Thus, there is a need for improved instrumentation to characterize thermal and mechanical induced deflections of motion picture film as it is transported, illuminated and projected in the gate of a motion picture projector.
The need is met according to the present invention by providing apparatus for measuring deflection of a film in a film gate having an aperture illuminated by a primary light source having an output spectrum, that includes:
a) a reflective photonic probe mounted in the film gate, the reflective photonic probe having at least one optical fiber;
b) a measurement light source coupled to a reflective photonic probe and emitting a wavelength of light outside of the primary light source""s spectrum;
c) a photodetector coupled to the reflective photonic probe;
d) a narrow pass optical filter located between the photodetector and the reflective photonic probe, the narrow pass filter passing the light from the measurement light source and blocking the light from the primary light source; and
e) signal processing electronics connected to the photodetector for producing a signal representing the motion of the film perpendicular to the plane of the film.
Advantages
The present invention enables the design of improved motion picture projection systems, film scanners and film recorders. Use of the instrumentation will enable better construction, alignment and calibration of motion picture projectors, film scanners and film recorders. In addition the same type of instrument can be utilized in the field for maintenance, repair and calibration of motion picture film projectors, film scanners and film recorders. The instrumentation can also be integrated into the motion picture film projector, film scanner or film recorder to provide active monitoring and real time feedback to servo control systems to maintain in-spec focus.